The invention relates generally to test equipment useful in diagnosing internal combustion engines. The invention relates more specifically to a signal detector apparatus that can detect coil ignition signals from a coil-on-plug device.
Internal combustion engines of the type commonly used in motor vehicles operate by igniting combustible gases in one or more cylinders using assistance from an ignition coil. The ignition coil has two windings: a low-voltage primary winding, and a high-voltage secondary winding. The windings cooperate to transform 12 volts D.C. from the battery into high voltage of 4,000 volts or more that is used by the spark plugs to ignite the air-fuel mixture inside the cylinders.
In a multi-cylinder engine, a distributor is used to couple ignition coil voltage to a plurality of spark plugs. The ignition coil output voltage is coupled to the center of the distributor. The distributor sends spark voltage to each spark plug at the proper time in synchronization with the cylinder combustion cycle.
Newer electronic ignition systems eliminate the distributor but have multiple coils. Each coil fires one or two cylinders at the same time. For example, a V-6 engine could use three ignition coils. In such a xe2x80x9cwaste spark-typexe2x80x9d ignition system, half the time, a spark is sent to a cylinder on an exhaust stroke when the spark is not needed. Nevertheless, waste spark design is an improvement over the distributor-type ignition system because it provides more accurate ignition timing. This higher accuracy results in more horsepower and lower exhaust emissions. A disadvantage of waste spark design is that an engine control computer cannot make cylinder-to-cylinder variations in the ignition timing. Rather, it has to change timing for two cylinders at a time.
In response to this issue, manufacturers have begun using xe2x80x9ccoil-on-plugxe2x80x9d ignition. For example, the 5.7-liter LS1 V8 engine of General Motors features a multiple coil ignition system having one coil per cylinder. Eight coil and driver module assemblies, fired sequentially, are mounted on the valve covers. Short secondary wires carry the voltage to the spark plugs just below the coil/driver module assemblies. Some manufacturers call this design xe2x80x9ccoil-near-plug,xe2x80x9d xe2x80x9ccoil-by-plug,xe2x80x9d or xe2x80x9cdistributorless electronic ignition.xe2x80x9d
Coil-on-plug ignition has numerous advantages. The system puts out very high ignition energy for plug firing. Because there are no wires or other connections from the coil to the plug, little or no energy is lost to connection resistance. Also, since firing is sequential, as opposed to waste spark, no energy is lost to the waste spark gap. It allows the vehicle computer to vary ignition timing for each cylinder, which improves power and reduces emissions. It provides simplified wiring and simplified diagnosis of problems.
Coil-on-plug ignition also enables compliance with current U.S. Government onboard diagnostic (OBD-II) regulations. These federal regulations specify that a vehicle computer must monitor for possible cylinder misfires that could be caused by a fault in the ignition or fuel-injection systems. Using coil-on-plug ignition, the computer can monitor the voltages produced in the secondary windings of the coil. Through computer analysis of these voltage signals, the computer can detect when a particular cylinder has misfired.
Also, a technician can determine which particular cylinder is at fault with the help of a diagnostic apparatus tool. Signal detectors (xe2x80x9ctest probesxe2x80x9d) are widely used in diagnosing and repairing defects in motor vehicles having internal combustion engines. A signal detector may be attached to an appropriate test point on a motor vehicle engine or other part under test. The signal detector detects an electrical or electronic signal at the test point and communicates the signal as input to a motor vehicle diagnostic apparatus, which generates and displays a waveform of the signal. Examples of suitable electronic digital signal analyzers or scanning tools include the Vantage(copyright) handheld diagnostic device, which is commercially available from Snap-On Diagnostics, San Jose, Calif.
FIG. 5A is a diagram of an ignition waveform 550 of the type generated by such signal analyzers, showing signal characteristics that are of interest in engine diagnosis, maintenance and repair. Generally, waveform is plotted on axes representing voltage (vertical axis) and time (horizontal axis). The characteristics that are primarily of interest are firing voltage, burn time, and burn voltage. Waveform 550 includes a firing voltage feature 552, burn time feature 554, and burn voltage feature 556. These features may be analyzed to determine whether an ignition coil or spark plug are operating correctly.
The Vantage(copyright) handheld electronic diagnostic device, with an additional electronic module that is commercially available from its manufacturer, can accept several different probes. In operation, a technician selects a desired test. A commonly conducted test identifies characteristics of the firing voltage and firing time of the ignition system. In this test, a detector end of a test probe is attached to the coil of the engine. Attachment may be direct, by conductive attachment to an electric signal point of the component under test, or indirect. The other end of the test probe is plugged into the diagnostic apparatus. The test probe communicates an electronic signal, representative of characteristics of the component under test, to the diagnostic apparatus. The diagnostic apparatus receives the signal, analyzes it, and displays a graph of the signal or recommendations for service.
However, conventional test probes are not adaptable to coil-on-plugs. There is no accurate, simple way to detect or obtain an ignition signal from a coil-on-plug device. One current approach for conventional engines involves attaching a diagnostic probe to the distributor coil, as exemplified by U.S. Pat. No. 3,959,725 (Capek, 1976). In the Capek approach, a single conductive probe is attached to the secondary coil of a distributor and wired to a positive signal input of an oscilloscope. A circuit is completed by coupling the negative signal input of the scope to chassis ground of the engine. In this approach, though, noise is a significant problem.
Further, there is no way to adjust the level of the signal input to account for differences in voltage output and other parameters of different coils and distributors. The Capek probe is prone to overloading or saturating the input circuitry of the oscilloscope or other test device. This causes the device to display inaccurate signal waveforms and can damage the device. Some oscilloscopes can be used to address this problem by adjusting gain controls that attenuate the input. However, modern handheld signal analyzers normally do not have such gain controls and require input signals to have an amplitude within a predictable narrow range.
Accordingly, there is a need in this field for an ignition signal detector or test probe that operates with coil on plug devices.
There is a particular need for a signal detector or test probe that can accommodate different coil on plug assemblies offered by different part manufacturers. Specifically, there is a need for a signal detector that provides a signal level that can be attenuated for different coil on plug assemblies to prevent overload of external test equipment and that provides a signal substantially free of noise.
The foregoing needs and objects, and other needs and objects that will become apparent from the following description, are fulfilled by the present invention, which comprises, in one aspect, an apparatus for detecting motor vehicle electric ignition signals from a coil on plug, comprising a first conductive planar layer and a second conductive planar layer separated by and affixed to a non-conductive substrate and adapted for mounting in close proximity to a coil of the coil on plug. In one feature of this aspect, the apparatus further comprises means for holding the substrate in proximity to the coil of the coil on plug and separated therefrom by a predetermined distance.
Another feature comprises a probe body adapted for interchangeably receiving one of a plurality of substrates, and means on the probe body for holding the probe body in proximity to the coil of the coil on plug and separated therefrom by a predetermined distance.
In one embodiment, the first layer is substantially rectangular, the second layer is substantially rectangular, and the first layer and the second layer have substantially equal surface areas. Alternatively, the first layer and the second layer have substantially different surface areas. In still another alternative, a difference in the surface areas of the first layer and the second layer is directly proportional to strength of the motor vehicle electric ignition signals.
In another aspect, the invention provides a diagnostic apparatus for use in analyzing ignition signals generated by a coil on plug. The apparatus comprises a signal detector comprising a first conductive planar layer and a second conductive planar layer separated by and affixed to an insulating substrate and adapted for mounting in close proximity to a coil of the coil on plug; a signal wire coupled to the first conductive layer and coupled to a signal input of a digital signal analyzer; a ground wire coupled to the second conductive layer and coupled to a ground input of the digital signal analyzer.
According to another aspect, the invention provides a method of measuring electric ignition signals of a coil on plug of an internal combustion engine. The method may involve holding a signal detector, comprising a first conductive planar layer and a second conductive planar layer which are separated by and affixed to an insulating substrate, in close proximity to a coil of the coil on plug; coupling a signal wire from the first layer to a signal input of an electronic digital signal analyzer; coupling a ground wire from the second layer to a ground input of the signal analyzer; and obtaining a measurement of the electric ignition signals using the signal analyzer based on detection of the electric ignition signals by the signal detector.
One feature of this aspect involves adjusting sensitivity of the signal detector by adjusting the relative size of the second layer with respect to the first layer.
In one specific embodiment, a signal detector comprises an insulating substrate having a first conductive layer on a first side and a second conductive layer on a second side. The first layer is coupled to a signal wire and the second layer is coupled to a ground wire. When the signal detector is held in close proximity to the coil of the coil-on-plug, ignition signals generated by the coil and passing to the plug are detected. The detected signals may be coupled to a signal analyzer for display and analysis.
One layer acts as a signal detector and the other layer acts as a ground plane. The ground plane reflects and absorbs a portion of the energy generated by the coil and thereby serves to attenuate the strength of the signal observed at the signal detector layer. The amplitude of the signal that is output by the signal detector may be adjusted to different coils having different output signal strengths by modifying the ratio of the surface areas of the first layer and the second layer. Noise is reduced through use of a differential signal.